A signal path is a logical grouping of multiple inputs, outputs, or busses that has a single name and (channel) format. The I/O Setup lets you create, define and name paths according to the configuration of your studio and the needs of each project.
Paths in Pro Tools tracks and I/O Setup include
main paths and
sub-paths.
Main paths are logical groupings of inputs, inserts, busses, or outputs. For example, a master stereo output path will include both its left and right channels.
A sub-path represents a signal path within a main path. For example, a default stereo output bus path consists of two mono sub-paths, left and right. Mono tracks and sends can be routed to either mono sub-path of the stereo output bus path.